1. Field of the Invention
This invention relates generally to a wind tunnel for aerodynamic wing study and, in particular, to an air flow system which bypasses or bridges over a balance or force transducer in a model airplane being tested.
2. Description of the Prior Art
Many of the forces and moments to which an aircraft is subjected by the air flow cannot be accurately determined by purely theoretical calculations. The aircraft designer must, therefore, have recourse to experimental aerodynamics, which from the earliest days has contributed much to the progress made in aeronautical science.
One of the most important experimental aids is the wind tunnel, a device whereby the reactions of a carefully controlled airstream on scale models of airplanes or their component parts can be studied. The first condition that a model for testing in the wind tunnel must satisfy is that of geometric similarity with a full-scale prototype. In addition, certain other important conditions relating to flow conditions and velocity must be satisfied (e.g., Reynolds number; mach number; etc.) to enable valid measurements to be performed on the model. The Reynolds number is a correction factor applied to the analysis of the flow around the model. It corrects for the scale effect resulting from the difference in size between the model and the prototype.
When the fluid flow around the model is the same as that around the prototype, there is dynamic similarity. For complete similarity between the full-scale airplane and a model, e.g., one-tenth its linear size, the air velocity in the wind tunnel would have to be ten times as high as the speed for which the airplane is to be designed. For high-speed aircraft, this would require impracticably high wind velocities in the tunnel and impracticably strong models to withstand the high pressures associated with such velocities. For these reasons, the tests are usually made on models at Reynolds numbers well below those for the full-scale conditions.
In the interpretation of the results, due allowance is made for this difference in dynamic conditions. Various models and devices are employed for performing the measurements of the forces, moments, torques, and pressures to which the models, attached to special balances or rigidly supported, are subjected in the wind tunnel.
With the increasing demand for full configuration wind tunnel data, scale models of aircraft now also require the simulation of engine propulsion. The more popular methods for engine simulation use compressed air. However, it is a problem in the prior art to use an air feed line that will have little or no effect on the accuracy of the force transducer.
A model airplane in a wind tunnel is usually mounted on a support strut of some kind, such as a sting, which also acts as the ground side or non-matrix part of the force transducer. The model airplane itself is mounted on the so-called live side or matrix part of the force transducer.
In most conventional wind tunnels, a balance is mounted inside the fuselage of the model airplane. The balance is a force-measuring transducer of the strain gauge type positioned in front of the sting. The transducer is a device that is actuated by power from one system and supplies power usually in another form to a second system.
In the prior art, internally pressurized bellows are positioned at the front of the balance. This arrangement has been found to cause tremendous force interactions which require corrections to data measured during the course of testing the model airplane in the wind tunnel.
Examples of such wind tunnel balances of the strain gauge type in which internally pressurized bellows are used are the prior art arrangements shown in U.S.S.R. Pat. No. 649,970 to Vitushkin et al., U.S. Pat. No. 4,074,567 to Horanoff, U.S. Pat. No. 3,878,713 to Mole, U.S. Pat. No. 3,233,452 to Jones, U.S. Pat. No. 3,159,027 to Curry, and U.S. Pat. No. 3,019,643 also to Curry.